In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, there is an increased usage of Wireless Local Area Networks (WLANs) based on the IEEE 802.11 family of standards. The IEEE 802.11 family of standards relates to communications standards for wireless local area networks. The maximum physical (PHY) layer bit rates of 802.11 have evolved from 2 Mb/s in IEEE 802.11-1997 to several Gb/s available with the current most amendment as represented by IEEE 802.11ac. Currently, IEEE 802.11n is a commonly used WLAN standard with support for multiple-input multiple-output (MIMO) compliant communications technologies, having a 40 MHz bandwidth and a maximum throughput in excess of 100 Mb/s.
A typical WLAN deployment comprises a number of network nodes, referred to as access points (APs), and a number of wireless end-user transceiver terminals, referred to as stations (STAs), associated with one of these APs. An access point and the associated STAs are referred to as a basic service set (BSS). Within a BSS, channel access is typically performed in a distributed manner using a distributed coordinated function (DCF) or a variety thereof, such as Enhanced Distributed Channel Access (EDCA). One main feature of DCF and its descendants is that it is based on carrier sense multiple access with collision avoidance (CSMA/CA), meaning that a STA senses the channel and only if the channel is sensed to be idle is the STA allowed to transmit. DCF and CSMA/CA is as such well known by the person skilled in the art.
It is possible that two or more APs may generate interference to each other in a geographical area. When the number of APs per area unit is relatively small, it is often possible to allocate different channels (frequencies) to APs that are within mutual range. In this way the APs will not interfere with each other. However, when the deployments become denser, the same channel may have to be reused such that APs and STAs using the same channel may interfere with each other. This kind of interference is commonly known as co-channel interference. Issues such as co-channel interference may also be expected to increase when wider channel are used since this implies that a smaller number of non-overlapping channels is available.
Situations with APs and associated STAs that interfere as described above is commonly referred to as overlapping BSS (OBSS). Co-channel interference is an issue also in cellular systems such as Long-Term Evolution (LTE) based communications networks developed by the 3rd Generation Partnership Project (3GPP), but because of the CSMA/CA mechanism its effect may be considerably worse for WLANs since WLANs need to cater for unlicensed/uncoordinated deployments.
The OBSS may effectively lead to that many of the BSS will not carry any traffic because the channel is sensed being busy due to traffic in another BSS. Thus, the channel is essentially time-shared between different BSSs, potentially leading to poor system performance.
A BSS may actually be quiet (i.e., in a state where no devices in the BSS transmit) although the sensed signal level is so low that successful transmission would have been possible. As a means to counteract the situation that the channel is sensed busy (and by that the transmission is deferred), although the signal power is so low that success transmission would have been possible, the threshold for where the channel is declared as busy may be increased. This is as such known in the art and sometimes referred to as dynamic sensitivity control (DSC).
Although DSC under certain circumstances may yield large improvement in terms of spectrum efficiency (as compared to when DSC is not used), DSC does not at all address the issue in WLAN that too much interference is generated due to the lack of power control.
On the other hand, in cellular systems, such as LTE, power control is used to minimize interference caused to other devices as well as to save power. Since the uplink (UL) and the downlink (DL) in cellular systems are using different resources, either in time (in case of time-division duplexing, TDD) or in frequency (in case of frequency-division duplexing, FDD), it may be ensured that different user equipment (UE) do not interfere with one another (in case of TDD there is a need to synchronize the radio base stations to avoid UL and DL to overlap and create interference between radio base stations and between UEs attached to different radio base stations). This means that when a UE is on the edge of network coverage for a radio base station the UE may still transmit at maximum power without other UEs being severely interfered.
However, for WLAN, because the same resources are used for both UL and DL, two STAs that are close to one another but operatively connected to different APs may severely impact each other. This impact may occur because of the above mentioned issue with CSMA/CA, but it may also occur because the carrier-to-interference ratio (C/I) becomes too small in case of strong interference.
One existing mechanism for handling issues with OBSS is based on fractional CSMA/CA and TPC for Interference Mitigation as presented in “Advanced power control techniques for interference mitigation in dense 802.11 networks” by Oteri, O. et al in the 16th International Symposium on Wireless Personal Multimedia Communications (herein after denoted Oteri). This mechanism may be used to coordinate transmissions between neighbouring BSSs to limit the interference experienced or caused by BSS-edge STAs to improve the overall throughput. This is achieved by limiting the transmission power in a neighbouring BSS when scheduling data to a BSS-edge STA. Effectively, STAs are divided into different groups. Associated with a group is a set of time slots when the channel may be accessed and a corresponding transmit power level. STAs that are close to the AP, denoted center STAs, are allowed to try to access the channel at all times, but are restricted to use a lower transmit power. STAs that are close to the edge of coverage, denoted edge STAs, are allowed to use higher output power, but are restricted to use the channel during certain time slots. By coordinating the use of the high power users such that the time slots when the STAs are allowed to transmit are not overlapping, or overlapping as little as possible, an improved energy efficiency is reported. Although the focus in Oteri is on energy efficiency, a gain in spectrum efficiency may be obtained due to the improved interference handling.
Another existing mechanism for handling STAs, as disclosed in “A Single-Channel Solution for Transmission Power Control in Wireless Ad Hoc Networks”, by Muqattash, A., and Krunz, M., in the Proceedings of ACM MobiHoc, 2004 is called POWMAC, according to which the sending STAs adjust the transmission powers of data packets to allow for some interference margin at the receiving STAs. Information about this interference margin is included in the CTS packet and is used to limit the transmission power of potentially interfering STAs in the vicinity of a receiver, rather than muting such STAs. Multiple interference-limited transmissions in the vicinity of a receiver are allowed to overlap in time. This mechanism does not explicitly address issues with OBSS problem but instead targets the spatial reuse in a mobile Ad-Hoc networks.
However, there is still a need for improved handling of overlapping basic service sets in wireless communications systems.